Research shows that insects that can resist the controversial pesticide DDT have an added advantage over non-resistant insects, even when they are not being sprayed with the chemical.

The finding, published in Current Biology today (9 August), not only has implications for dealing with pesticide-resistant insect pests, but could also shed light on the way disease-causing organisms develop resistance to drugs and antibiotics.

The study overturns the theory that pesticide resistance comes at a cost, making resistant insects less able to compete with non-resistant ones when the pesticide is absent.

"We found that DDT resistance in fruit flies not only carries no cost, but also confers an advantage when inherited through the female," says lead researcher Richard ffrench-Constant, of the University of Bath, United Kingdom.

As well as being able to survive exposure to DDT, the resistant insects also produced three times as many eggs as non-resistant fruit flies.

Previously, it had been thought that stopping the use of a pesticide would help rid an area of pesticide-resistant insects, by removing the factor that gives them an advantage over non-resistant members of the same species.

There are now concerns that this finding may also be relevant for doctors who prescribe antibiotics to cure infections.

"These results are important for the use of any drug, pesticide or antibiotic as they suggest that resistance will not always go away when we do not spray or prescribe antibiotics," notes ffrench-Constant.

The findings come at a time when many countries, especially those in sub-Saharan Africa, are considering re-introducing DDT to combat malaria.

DDT is widely banned because of concerns about its effects on human health and the environment but some countries still use it to control malaria-carrying mosquitoes.

In Kenya, two leading research institutions — the Kenya Medical Research Institute (KEMRI) and the International Center for Insect Physiology and Ecology (ICIPE) — disagree over whether DDT should be re-introduced.

ICIPE argues that the health and environmental risks of reintroducing DDT are considerable and that the East African region as a whole would suffer if the ban were lifted.

KEMRI, on the other hand, says Kenya's decision to ban DDT in 1990 was taken hurriedly and without adequate data.

KEMRI's position reflects that of the World Health Organization, which champions the continued used of DDT to control insects that transmit parasites to people, under the Stockholm Convention on Persistent Organic Pollutants.

This convention, among other things, recognises "urgent and immediate need of many of the malaria-endemic countries to maintain their reliance on the use of DDT for indoor residual spraying to control insect vectors, particularly malaria vectors, until viable, effective and affordable alternatives are found."